Dendritic cells (DCs) are professional antigen-presenting cells with a central role in adaptive immune responses. They are specialized for the uptake, processing and presentation of antigen to T-cells. By guarding mucosal barriers and transporting antigens to lymphoid organs, they critically control the balance between protective immunity and immune tolerance. Pathogen- induced inflammatory responses lead to the emergence of DCs at the infection site and draining lymph nodes. A significant question is what signals regulate the development and function of DCs at inflammatory foci. Recent research suggests a central role for inflammatory monocytes as major precursors for de novo DC formation during infection. This R21 application presents our plan to understand the role of Notch signaling in the differentiation and function of monocyte-derived DCs (Mo-DC) in chronic inflammation. As outlined in our research approach, we recently identified a unique population of Mo-DCs that have an enhanced ability to undergo cell-cell fusion, a hallmark of chronic inflammation. These cells are absent in steady-state lymphoid organs, however, they are present under inflammatory conditions. We have identified the precursors of these inflammatory DCs in the bone marrow and shown that differentiation in vitro is Notch-dependent. In this proposal, we will address the functional significance of these Notch-dependent Mo-DCs and the role of Notch signaling in the differentiation and function of these cells. We hypothesize that these cells are endowed with the ability to prime effector and/or regulatory T-cell responses and that Notch signaling critically drives both the differentiation and function of these cells. Togethr, these studies should not only lead to an improved understanding of chronic inflammatory disorders but will also identify novel therapeutic strategies for treatment.